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Did marine sponges trigger the ‘Cambrian explosion’ through ‘ecosystem engineering’?

21.09.2017

Sponges as “water cleaner” raised dissolved oxygen concentrations of seawater, which was necessary for the appearance of animals 550 million years ago

About 550 million years ago, almost all of todays’ living animal phyla appeared on Earth within just a few million years. This expansion of metazoans is also known as the ‘Cambrian explosion’ and marks the beginning of the “Phanerozoic”, the era of ‘visible life’.

Microscopic image of a siliceous sponge spilule, preserved in a Cambrian shale. The width of the picture is 5 mm.

GFZ/M. Tatzel

The causes of this most dramatic ecosystem change on our planet are still disputed. Geologists know that during this time, the concentration of free oxygen significantly increased in the atmosphere and the oceans. As metazoans (multicellular animals) need oxygen for breathing, this rise in oxygen levels was fundamental to their emergence. But what were the causes of this essential oxygen increase?

Scientists of the German Research Centre for Geosciences GFZ could show for the first time that marine sponges induced changes in the marine carbon and phosphorous cycles that caused a rise in seawater dissolved oxygen levels, providing the basis for the further expansion of metazoans. The study is published in the journal “Nature Communications”.

The GFZ-geochemists provided the evidence by means of a new method using stable silica isotopes – these subgroups of chemical elements that have different atomic masses. Siliceous sponges live on the seafloor and leave behind silicon-rich spicules (see photo) of their skeleton upon their death. However, these spicules are rarely preserved when old seafloor turns into rock.

Michael Tatzel and Friedhelm von Blanckenburg analysed silicon stable isotopes using a modern mass spectrometer to determine the abundance of siliceous sponges in sediments. They did so because silicon from these spicules is retained in shales and cherts (flintstones) and is enriched in “light” silicon-28 relative to silicon-30.

“Our reconstructions reveal an increasing abundance of siliceous sponges in sediments that were deposited between the Precambrian and the Cambrian on the continental slope of todays’ Yangtze Platform in South China”, says lead author Michael Tatzel.

But how does increasing sponge abundance relate to oxygen? Michael Tatzel investigated a multitude of geochemical indicators that react sensitively to the amount of oxygen dissolved in seawater. He found that simultaneously with sponges, the concentration of dissolved oxygen as well as the amount of organic carbon deposited in sediment increased. These fundamental changes resulted from the way sponges live.

Sponges extract organic carbon from seawater for feeding and could thus shift the oxidation of organic carbon to depth or even decreased oxidation. This change to the carbon cycle initiated a chain reaction: the increasing oxygen concentration promoted phosphate deposition and thus reduced phosphorous concentration in seawater and thereby the growth of algae that consume oxygen from seawater upon their death.

“This is, in our view, the first substantial evidence for the hypothesis that sponges functioned as ecosystem engineers and raised oxygen levels in seawater”, says Michael Tatzel. There is a good chance that this sponge-induced increase in oxygen levels exceeded the minimum requirement of multicellular life forms and thus triggered the ‘Cambrian explosion’.

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